Vane pumps are already being employed for delivering sausage meat from hoppers into filling tubes via which sausage casings are filled with the sausage meat. Such vane pumps are already described, for example, in the EP 0432388 B2 and the EP 1040758 A1. Vane pumps consist of a pump case and a rotor eccentrically arranged therein which can be caused to rotate. Vanes held so as to be radially movable are arranged in the rotor and form delivery cells together with the wall of the pump case, the bottom and the lid of the case and the outer surface of the rotor and cooperate in a sealing sense. The pump case has a meat inlet and a meat outlet. The area downstream of the inlet to downstream of the outlet corresponds to the pressure area of the pump. In the outlet area, the respective vane cell is in communication with the outlet, so that the pasty mass can be, for example, pushed from the vane cell into the filling tube. In the sense of rotation downstream of the outlet, the sealing area is provided which separates the pressure area from the suction area.
In former solutions of vane pumps, the pressure area is separated from the suction area by a sealing gap generated between rotor and pump case. The sealing gap resulting between the rotor and the case is in the range of only a few hundredths of millimeters. The sealing gap and the sealing function here depend on the geometry as well as the existing tolerances of the delivery mechanism parts. In addition, wear and different performance characteristics, such as delivery pressure, product consistency, etc. have a disadvantageous effect on the uniformity of discharge.
In case of high pressures in the pressure area, the sealing gap increases, e.g. as a result of elastic deformations. Due to the fact that the pressure drop in the sealing gap is proportional to the third power of the gap width s, i.e. Δp˜s3, varying sealing gaps have enormous influence on the leakage flows and thus on the portioning accuracy of the delivery mechanism. In addition, the sealing effect depends on the effective length of the sealing area.
When the sealing effect is decreasing, the leaking mass flowing from the pressure into the suction area increases. This leaking mass can thereby penetrate the area of a vacuum extension provided in the pump case and flow further towards the vacuum pump. Meat deposits in the area of the vacuum extension or in the vacuum channel to the vacuum pump, respectively, can lead to a clogging of the channel or to a damage of the vacuum pump.
Due to the gap width problem, the degree of the evacuation of the product can neither be purposefully influenced. That means, the leakage amount and thus the portioning accuracy depend on the wear condition of the delivery mechanism components and on various performance characteristics, such as delivery pressure, product consistency, etc. With increasing wear and thus decreasing sealing function, expensive delivery mechanism parts have to be replaced, such as rotor, pump case, pump vane, etc., to ensure an adequate sealing function.